There are so many basic milestones that Emma Raducanu has not yet recorded in a professional tennis career that only began in full three months ago. She has never been a direct entrant to a grand slam main draw, she is yet to play a tour-level three set match and she has not even won a match at a WTA tour event.

Yet sometimes a special player comes along and renders convention irrelevant. After three weeks and one of the most astonishing breakout runs in living memory, Raducanu marked herself as a grand slam champion for ever. She ended the US Open where she started it: fearlessly dominating from inside the baseline as she defeated Leylah Annie Fernandez 6-4, 6-3 in a match of the highest intensity to win the title without dropping a set.

Related: Emma Raducanu v Leylah Annie Fernandez: US Open women’s final – live!

This is just Raducanu’s second grand slam main draw appearance and no woman in the Open era had ever won in so few attempts. Along with that distinction is a seemingly endless list of records: Raducanu is the first British woman to win a grand slam title since Virginia Wade’s Wimbledon victory in 1977. She is the first qualifier, man or woman, to win a grand slam title. At 18, she is the youngest grand slam champion since Maria Sharapova in 2004.

From the very beginning of the first slam final between teenagers in 22 years, both players refused to be affected by the moment. Throughout the first set they wrestled the baseline from each other, defended with elite movement and imbued every game with sheer intensity. Raducanu started the match by controlling the early exchanges by constantly changing direction off both wings, and she set the tone by snapping a cross-court backhand winner in the opening game.

After holding serve, Raducanu immediately imposed herself on return, breaking on her sixth break point. But Fernandez responded with her own suffocating return game, breaking back on her fourth break point for 2-1. After 22 long minutes of sheer intensity, they had played only three games.

As she faced her toughest opponent of the championships, Raducanu was at times visibly frustrated by her level as Fernandez’s defence eked out errors that the Briton’s other opponents could not. But as the set wore on, she again began to dominate the key points with her firepower off both wings.

Raducanu made her run at 5-4 with a series of suffocating returns, snapping a cross-court backhand return winner to bring up double set point. Fernandez saved both, including with a bold series of forehands and then Raducanu missed a third. But Raducanu remained on top of the baseline; she drilled a winning inside-out forehand to bring up a fourth set point and then dispatched another big forehand to take the set.

The momentum continued to move Raducanu’s direction as she marched up to lead 6-4, 1-0 and 0-40 on Fernandez’s serve. But with her back to the wall, the Canadian stepped forward and swung freely. She saved all three and then used the momentum to break serve for 2-1. Raducanu responded immediately, calling on her immense return of serve and breaking back with an angled cross-court return winner. After holding serve, she played a ballistic return game and broke again for 4-2 with a running forehand down-the-line passing shot winner.

Two match points came soon after for Raducanu at 5-2, but both times she struck nervous forehands and missed. Fernandez, battle-hardened by four consecutive three-setters, began a last ditch charge. She held serve and then she had the momentum as she attacked Raducanu’s serve. At 30-30, she had her opponent on the ropes and nailed a winning forehand, but as the Briton slid towards a defensive backhand she grazed her left knee as she slid.

Raducanu was bleeding and so with the score at 6-4, 5-3 and a break point down in the biggest match of her life, she was forced to take a medical timeout. Fernandez, who had the momentum, was unsurprisingly displeased and when they finally resumed, she immediately shanked a forehand on the break point.

But then she generated another break point, asking Raducanu a last question. Under grand slam final pressure that has suffocated so many great players over the year, Raducanu’s reaction said all that needs to be known about her grit and composure. She saved the second break point on the front foot with an overhead winner at the top of her reach, then she drilled a backhand down the line to bring up match point. She served it out with an ace, in front of a crowd that included Wade.

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After congratulating Fernandez and her team on their run and thanking her own entourage and supporters, the victor said: “It was an incredibly difficult match but I thought the level was extremely high. I hope we play each other in many more tournaments and hopefully finals.”

She added of the final game: “Leylah is always going to play great tennis and always going to fight. That’s just the competitor she is and that’s why she’s here in the final. I knew that I’d have to dig deep and I fell somehow. I thought that would throw myself off balance because I would have to serve. I was just praying not to double fault, really. We got through, just staying in the moment, focusing on what I have to do, my process in the moment really helped in those tough times.”

When Raducanu arrived in New York three weeks ago, there was some caution about her physical condition after an intense few weeks on the road in what still is her first ever long trip away from home for competition. Her flights were booked for the end of the qualifying week because a loss in any round would have been a totally normal result and a valuable learning experience.

Related: Emma Raducanu salutes beaten Leylah Fernandez on ‘incredible’ tennis

And yet, after just her fourth tour level event, she leaves New York as the champion. After starting the summer ranked 366th and the 12th-ranked British player, Raducanu is now the world No 23. Her career earnings at the beginning of the summer stood at $35,185 yet she has earned $2,500,000 (£1.8m) in three weeks.

With every new challenge she has faced, she has learned and adapted in the moment, unlocking new qualities against some of the best tennis players in the world. In a few small weeks she has improved as much as some do in years and there is so much more to come.